In general ionising radiation is considered to be radiation within the energy range 5 KeV to 6 MeV and includes gamma rays, x-rays, beta-rays, alpha-rays and neutron beams. Devices for detecting ionising radiation are well-known for radiological protection and metrology, such as in health or nuclear physics as well as national/homeland security and anti-terrorist applications. The devices are one of two types, either passive detectors or electronic-based active detectors.
Passive detection systems use film (film-badges), thermo-luminescent detection (TLD) or photochromatic technologies (PC) as detector materials. Common to these detector technologies is that they register the presence of ionising radiation by a change of state. For example, a film exposed to ionising radiation goes dark when developed, TLD materials emit light when heated having previously been exposed to ionising radiation and PC materials change colour when irradiated with ionising radiation. However, the change of state of these materials requires special processing in order to be determined, for example developing the film or heating the TLD material. Consequently, only an historic monitoring and evaluation of radiation exposure can be obtained. It is not possible to achieve real-time monitoring and evaluation. Since no direct real-time monitoring or analysis is possible it is therefore necessary to infer what type of radiation exposure caused the change of state. Although such inference can be drawn based on experience, nevertheless it is not possible to precisely determine what type of radiation (spectroscopic information) has been sensed nor an estimate of radiation dose which takes into account such information. Additionally, known passive detection systems generally have poor sensitivity to ionising radiation.
Active detectors may be based upon silicon technology and generally comprise one, two or three PIN-diodes, each PIN-diode having a preset threshold level to signal an alarm relating to a minimum energy level of incident radiation.
If more than one PIN-diode is used then different threshold levels may be preset corresponding to different radiation and energy levels thereby providing crude spectroscopic analysis of incident radiation. However, silicon has poor sensitivity to ionising radiation since it does not have a high atomic number (Z), therefore there is inefficient conversion of the incident radiation to electric current and devices using such technology suffer from poor signal to noise ratio.
Another drawback of known active detectors is that the electronic signals are generated remote from the detector substrate, leading to signal losses and signal mis-shaping due to the impedance of connecting wires and circuitry.
Furthermore, the spectral resolution of known devices can be poor, particularly if a low grade detector material is used. This is because low grade detector material can affect the amount of charge generated in a photon interaction charge generation event, and also may affect the transport properties of the detector material which may reduce the amount of charge that may be collected at an electrode. This leads to inaccurate measurement of charge and hence inaccurate estimation of the energy of the incident photon which gave rise to the charge generation.
Additionally, where the charge is generated in the detector substrate determines the amount of charge that is collected at an electrode, in particular the phenomenon of “charge trapping” reduces the charge that may be collected at the electrodes and the further away from an electrode that charge is generated the greater the possibility of charge trapping. This blurs the perception of the charge generated for a given radiation event and hence reduces isotopic spectral resolution.
Various solutions have been proposed to compensate for the deleterious effects described above.
One example of a known technique for addressing at least some of the problems described above is the Frisch Grid arrangement [1] in which a band of metal is placed around the outside of the sensor material block. The metal band provides a secondary bias electrode. A voltage is applied to this secondary electrode, which changes the electric field in such a way to inhibit the passage of holes towards the cathode and hence restrict charge collection signals to those generated from electrons. Such an arrangement has been know to provide 2% spectral resolution FWHM (Full Width Half-height Maximum) at 662 KeV.
A second example is a coplanar grid arrangement of electrodes in which charge collection electrodes are separated by non-charge collecting electrodes. The charge collection electrodes collect electrons and also induce a charge on the non-collecting electrodes. By post-processing of signals the effect of induced charge on collection electrodes due to charge collected on other charge collection electrodes may be compensated for.
A third method is a ballistic compensation method. A respective anode and cathode electrode are placed on opposite sides of a block of sensor material, and the charge collected at the anode and cathode electrode compared to determine the depth of interaction of the incident radiation. This depth information is used to compensate for depth dependent charge collections effects. Thus improving the accuracy of the estimate of the energy of the photon interaction event causing the generation of the charge.
Embodiments of the present invention were devised with the foregoing in mind.